NITROGEN DOPED CARBON CATALYSTS FOR ELECTROCHEMICAL OXYGEN REDUCTION REACTION

Energy issues have a prominent position in the economy and development at a world level. The automotive sector is at present based almost entirely upon the internal combustion engine and, therefore, is highly dependent on petroleum derived fuels. Polymer electrolyte membrane (PEM) fuel cells are clean and attractive power generators for transportation and mobile applications due to their high energy conversion efficiency, low operating temperature and low environmental impact. However their practical use continues to be hindered by the high cost of the platinum based catalysts that are needed to improve the slow kinetics of the electrochemical oxygen reduction reaction (ORR) at as low operating temperatures as T ~ 80-120 °C. To reduce catalysts cost two different approaches are used, the first one being to reduce platinum loading using highly dispersed metal particles (nanoparticles, films) and platinum alloys; the other one consisting of using metal-free or non-precious metal loaded (Fe, Co) nitrogen doped carbons. Along this line Co/N/C and Fe/N/C catalysts were synthesized and their activity for ORR was investigated manly with electrochemical methods (Thin Film Rotating Disk Electrode, TFRDE, technique and Membrane Electrolyte Assembly, MEA). Others methods like potentiometric titrations, SEM-EDX analysis and thermal analysis were used to characterized carbon support and catalysts. The investigation of commercial Pt/C catalyst was performed with the RDE, varying the electrode structure and component concentrations, catalyst layer thickness, overlayer Nafion film thickness and Nafion concentration inside the catalyst layer. This latter parameter was found to affect the reagents diffusion (O2, H+) inside the catalyst layer. Results were used to outline some criteria that have to be obeyed to obtain reliable RDE data on supported catalysts. Nitrogen doped carbons were synthesized by amine introduction onto high surface area commercial carbons. A synthesis based on stoichiometric amination of acid oxygen carbon groups, produced un-effective catalyst for ORR, also using different amine compounds, metal ions and carbon support. An effective carbon modification was obtained by pyrolysis of a completely flooded carbon precursor with a liquid amino compound in which metal ions are dissolved. The effect of Nafion on the diffusion current inside the catalyst layer is stronger in Pt-free than Pt/C catalyst, with different trends of dependence of the Nafion/catalyst ratio. Effective catalyst were also obtained by carbonization of a hard templated glucose/histidine precursor with different Fe/Cu ratio. Metal loading and reagent molar ratio effects were investigated by BET, XPS and electrochemical methods.